The B. subtilis bacteriophage, 1029, has twelve appendages of a processed form of gp12 which are involved in attachment to the cell. It is not known what the oligomeric; state of these "fibers" is. It has been long thought that they were dimers, but most bacteriophage fibers have turned out to be trimers. Mass measurements by the STEM of purified gp12 "fibers" should resolve this question. The purified gp12 protein is 86 kDa by SDS-gel analysis and the native form is a dimer or trimer by sedimentation analysis. In a prelimins ry study, the fibers looked very nice but (inadvertently, because of a new filament) the dose was too high to obtain meaningful mass measurements. The experiment was repeated, along with a mutant (shorter) gp 12. The mass measurements were a little high for a dirner, perhaps because of some salt or buffer, but not nearly close to being a trimer.